Although the Internet has had great successes in facilitating communications between computer systems and enabling electronic commerce, the computer systems connected to the Internet have been under almost constant attack by hackers seeking to disrupt their operation. Many of the attacks seek to exploit vulnerabilities of the application programs, operating systems, and other computer programs executing on those computer systems. One of the most destructive methods of attacking a computer system has been to modify portions of the operating system either on disk or in memory with software that may perform many of the same functions of the operating system, but also includes malicious functions. These modifications can be either to replace portions of the operating system or to add new programs that are automatically started by the operating system. Such software is referred to as “malware” because of its malicious nature. Once malware is installed on a computer system or host, the operating system is “infected” and the malware can control all aspects of the computer system. Such malware includes rootkits, Trojans, keystroke loggers, and so on. For example, the malware could intercept keystrokes that a user enters (e.g., a password) and report them to another computer system. As another example, the malware could be a worm that launches a self-propagating attack that exploits a vulnerability of a computer system by taking control and using that computer system to find other computer systems with the same vulnerability and launch attacks (i.e., send the same worm) against them.
To prevent detection, malware often uses advanced stealth techniques to make its resources such as executable files, data files (e.g., recorded keystroke files), and processes “invisible” or “hidden.” Once malware is started, it may intercept all queries (e.g., system calls) at a very low level (e.g., kernel mode) and use filtering to ensure that a chosen subset of its resources is never revealed to any query operations by any program running on the infected computer system. For example, a keystroke logger may intercept all requests from programs for file listings and delete the name of its log file from the list that is returned to the programs. The keystroke logger may even intercept requests for available disk space and add the size of the log file to the available disk space to make it difficult to detect the presence of the log file. The keystroke logger may also intercept all requests from programs for process related information and remove any information related to the keystroke logger process, task, thread, and so on. These stealth techniques can defeat both experienced system administrators who use common operating system utilities, such as WINDOWS Explorer, a directory list command, a registry editor, a task manger, and so on, to search the file system, configuration files (e.g., registry files), and process tables for suspicious entries, and commonly used malware scanning tools that are based on known malware file signatures.
Many different techniques have been used to help detect the presence of such malware; unfortunately, detection of some malware has proved to be difficult. One technique attempts to identify the presence of malware by the presence of an open port. Malware may install a backdoor so that the computer system can be accessed at a later time. The backdoor opens a port through which another computer system can gain access to the infected computer system. The technique can initiate a port scan from another computer system to detect the presence of an open port. If, however, the malware only opens a port at designated times, then the scanning may not detect the open port. Another technique may compare the files of the infected operating system with files of a non-infected or “clean” operating system. In particular, the technique may generate hash codes for the files of the infected operating system and compare them to hash codes of the clean operating system. However, since the malware may have total control over the computer system, it can provide the clean version, rather than the infected version, of a file to a program that is calculating the hash codes.
Because of the difficulty in detecting malware, some computer users have resorted to reinstalling an operating system onto a storage device that might be infected from a storage device that contains a copy of the operating system that is known (or thought) to be clean. Such reinstalling of an operating system is time-consuming. Moreover, such reinstalling is unnecessary if the operating system was not actually infected—although the user could not be sure that it was not infected.